Method and device for deionising cooling media for fuel cells

ABSTRACT

The present invention relates to a process for the deionization of a cooling medium in a fuel cell ( 11 ) circulating in a cooling circuit ( 20 ), in which the cooling medium is subjected to at least intermittent, but preferably continuous, electrochemical deionization. To this end, at least one electrode deionization cell ( 23 ), through which a diluate stream ( 27 ) serving as cooling medium and a concentrate stream ( 28 ) flow, is arranged in the cooling circuit. The concentrate stream ( 28 ) may be part of a secondary cooling circuit.

[0001] The present invention relates to a process for the deionizationof cooling media for fuel cells, and to an apparatus for carrying outthe process.

[0002] Fuel cells are devices in which a fuel, for example methanol,ethanol, hydrogen or corresponding mixtures, can be burnt in acontrolled manner using a combustion medium, for example pure oxygen,air, chlorine or bromine gas, with the reaction energy liberated in theprocess being converted not only into thermal energy, but also intoelectrical energy. Fuel cells have been employed for several decades forproducing electrical energy, in particular in space travel. Owing totheir high efficiency, their low or zero emission of pollutants andtheir low production of noise during operation, the interest in the useof fuel cells in other areas too has increased greatly in recent years.Particular mention should be made here of the motor vehicle and powerstation sectors.

[0003] Fuel cells are typically classified by the nature of theelectrolyte which separates the anode and cathode chambers. Aparticularly interesting type of fuel cell which is particularlysuitable for use in relatively small power stations and for mobile use(for example as vehicle drive) is the polymer electrolyte fuel cell. Inthis type of fuel cell, an ion-conducting membrane is used aselectrolyte. A single solid-polymer fuel cell generally comprises aso-called membrane electrode assembly (MEA), in which an ion-conductivemembrane is arranged between a cathode and an anode. The ion-conductivemembrane here serves simultaneously as dividing wall and as electrolyte.Catalyst particles which promote the conversion reaction in the fuelcell are arranged at the interface between the electrodes and themembrane. The electrodes are typically in contact with porous currentcollectors, which in addition stabilize the electrode structure andallow the supply of fuel and combustion medium. Since the operatingvoltage of a single cell is normally less than 1 volt, most fuel cellsconsist of a cell stack in which, in order to produce a higher voltage,a number of individual cells stacked one on top of the other areconnected in series. The typical operating temperature of a polymerelectrolyte fuel cell is in the region of 100° C. Higher temperaturescan result in damage to the membrane. Since the electrochemical reactionbetween the fuel and the combustion media proceeds exothermically, thefuel cell normally has to be cooled so that the desired operatingtemperature can be maintained. Since a relatively large amount of heathas to be dissipated with only a small temperature difference to theambient temperature, liquid coolants of sufficiently high thermalcapacity are typically employed. Water-based coolants are thereforeparticularly suitable.

[0004] However, water-based coolants have the disadvantage that they maycontribute to corrosion in the metallic constituents of the coolantcircuit and of the fuel cell. In addition, a cooling medium which has acertain electrical conductivity represents a safety problem in the fuelcell stacks which are operated at relatively high voltage, for exampleat about 50 volts.

[0005] Since the electrical conductivity of an aqueous cooling mediumlikewise drops with decreasing ion concentration, it has already beenproposed to use deionized cooling media for fuel cells. For example,U.S. Pat. No. 5,200,278 and WO 00/17951 disclose arranging ionexchangers in the cooling circuit in order that the aqueous coolantremains substantially free from ionic impurities for a certain period.If deionized water is used as the coolant, this can simultaneously beused for moistening the reaction participants flowing into the fuel cellin order to ensure adequate hydration of the polymer membrane. However,a disadvantage of the known systems is that the ion exchanger becomesexhausted after a certain operating time and has to be replaced. This isconsequently associated with a high maintenance requirement and highcosts.

[0006] It is an object of the present invention to provide a process forthe deionization of the cooling medium for a fuel cell which enablessubstantially maintenance-free operation and avoids shut-down of thefuel cell caused by exhaustion of the ion exchanger.

[0007] We have found that this object is achieved by the process for thedeionization of a cooling medium in a fuel cell as claimed in thepresent claim 1. It is proposed in accordance with the invention thatthe cooling medium circulating in a first cooling circuit be subjectedto at least intermittent electrochemical deionization. With the processaccording to the invention, the cooling circuit in the fuel celloperates with virtually no maintenance. As soon as, for example, aconductivity sensor records an increase in the conductivity of thecooling medium, which corresponds to an increase in the ionconcentration, voltage can be applied to the electrodes of anelectrochemical cell arranged in the cooling circuit, which removes someof the ions from the cooling circuit. Use is preferably made ofelectrodialysis cells, which can be operated with or without ionexchangers. If ion exchangers are used, the corresponding cells are alsoknown as electrode ionization cells. In cells of this type, thedeionization of the medium and the regeneration of the ion exchangerstake place at the same time.

[0008] One or more heat exchangers are arranged in the cooling circuit.According to a variant of the invention, the first cooling circuit is atthe same time the only cooling circuit, and the heat exchanger orexchangers is (are) in contact, for example, with air or water oranother suitable cooling medium. However, the first cooling circuit mayalso, as primary circuit, be in thermal contact with a second circuit(secondary circuit).

[0009] According to a preferred embodiment of the process according tothe invention, the deionization of the cooling medium is carried outcontinuously during operation of the fuel cell.

[0010] Since lower residual conductivities of the cooling medium can beachieved on use of ion exchangers than in the case of pureelectrodialysis, use is preferably made of electrode ionization cells,and the cooling medium is passed through the cell as diluate stream.Electrode ionization cells are known per se and are used, for example,for the desalination of sea water. An electrode ionization cell of thistype may consist, for example, of a mixed bed of anion and cationexchanger resins. According to another variant, anion and cationexchanger resins are arranged in two separate chambers. The diluateflows through the ion exchanger packs, which are separated from theconcentrate stream by ion-selective membranes.

[0011] The diluate stream is advantageously cooled before thedeionization in order to keep the temperature of the solutions incontact with the ion exchanger components low. To this end, theelectrode ionization cell may, for example, be arranged downstream(based on the flow directions of the diluate) of the coolers or heatexchangers in the first cooling circuit.

[0012] According to a particularly preferred variant, the first coolingcircuit is designed as primary cooling circuit, with the depleteddiluate stream coming into contact with the corrosion-endangeredcomponents. The concentrate stream from the electrode ionization cellcan then be allowed to circulate in a second cooling circuit, thesecondary cooling circuit, and cooled in a primary heat exchanger. Thecooled concentrate stream can subsequently be used for cooling thediluate stream. The secondary circuit of the concentrate stream can havea water supply with which the water losses occurring in operation duringregeneration of the ion exchangers can be compensated. In this variant,the heat from the diluate stream, after leaving the fuel cell, ispreferably transferred to the secondary circuit containing theconcentrate stream via a primary cooler. The cooled diluate streamsubsequently passes through the electrode ionization cell. The heatedconcentrate stream is passed through the primary cooler and subsequentlyinto the electrode ionization cell, where it takes up the ions migratingout of the diluate.

[0013] The ionic conductivities which can be achieved in the depleteddiluate stream by means of the process according to the invention are,depending on the initial conductivity, usually less than 1 μS/cm. It iseven possible to achieve conductivities of less than 0.1 μS/cm.

[0014] The present invention thus relates in its most general form tothe use of an electrode ionization cell for the deionization of thecooling medium in a fuel cell.

[0015] The present invention also relates to a fuel cell unit having atleast one fuel cell and a first cooling circuit for the fuel cell,wherein at least one electrode ionization cell, through which a diluatestream serving as cooling medium and a concentrate stream flow, isarranged in the cooling circuit. It is possible to use a very widevariety of electrode ionization cells known per se (cf., for example,Ganzi et al. “Electrodeionization”, Ultrapure Water, July/August 1997).

[0016] The electrodes of the electrode ionization cells can be made ofsuitable materials, for example noble metals, in particular platinum,metal oxides or graphite. The cathodes may also consist, for example, ofsteel or nickel. The separation between the membranes is usually fromseveral hundred μm to a few cm. The current densities are dependent onthe residual conductivities of the solutions and can be from a few mA/m²to several A/m². In the case of continuous operation, the energyrequirement of an electrode ionization cell of this type is less thanone watt per liter of solution.

[0017] According to a variant of the invention, the chambers of theelectrode ionization cell do not contain ion exchanger packing. In thiscase, the cell is operated as a pure electrodialysis cell. However, theachievable residual conductivities are greater than in the case of acomparable electrode ionization cell containing ion exchanger packing.

[0018] However, ion exchanger packing is particularly preferablyprovided. The ion exchanger may consist, for example, of a mixed bed ofanion and cation exchanger resins which is delimited on the cathode sideby a cation exchanger membrane and on the anode side by an anionexchanger membrane. The diluate stream to be depleted flows through thepacking. The ion exchanger membranes are in contact on the side oppositethe ion exchanger bed with the concentrate stream, which is at the sametime in contact with the electrodes, between which the electric field isbuilt up. This variant offers the possibility of constructing a numberof diluate and concentrate chambers alternately in order to facilitategreater volume throughput for the same electrode surface area.

[0019] According to another variant, the diluate flows through thecation exchanger resin and anion exchanger resin in two separatechambers. The cation exchanger resin packing here is delimited on theone hand from the concentrate stream by a cation exchanger membrane andon the other hand from the anion exchanger resin packing by a so-calledbipolar membrane. At the bipolar membrane, protons are liberated on theside of the cation exchanger resin packing and hydroxyl ions on the sideof the anion exchanger resin packing. The anion exchanger resin packingis itself delimited from the concentrate stream by an anion exchangermembrane.

[0020] The concentrate stream preferably flows around the electrodes ofthe electrode ionization cell. If constituents of the concentrate streamare sensitive to electrode reactions, the electrodes can, for example,be screened by a simple ion-selective membrane in order that anodicallyor cathodically unstable components may also be present in theconcentrate stream. Thus, for example, glycols can be added asantifreeze component. The cooling medium may also comprise additionalcorrosion inhibitors, for example the orthosilicates described in thepatent application DE-A 100 63 951. The orthosilicates preferably havefour identical alkoxide substituents, in the form tetra(alkoxy)silane.Typical examples of suitable silicates are pure tetraalkoxysilanes, suchas tetramethoxysilane, tetraethoxysilane, tetra(n-propoxy)silane,tetra(isopropoxy)silane, tetra(n-butoxy)silane,tetra(tert-butoxy)silane, tetra(2-ethylbutoxy)silane,tetra(2-ethylhexoxy)silane ortetra[2-[2-(2-methoxyethoxy)ethoxy]ethoxy]silane. Said substances areeither commercially available or can be prepared by simpletransesterification of one equivalent of tetramethoxysilane with fourequivalents of the corresponding relatively long-chain alcohol or phenolby removal of methanol by distillation.

[0021] Particularly suitable cation exchanger membranes areperfluorinated membranes, for example Nafion®117, which is made byDupont. Water diffusing through the membranes is decomposed to formhydrogen and oxygen by application of an electric voltage to thegas-evolving electrodes. According to a further variant, use can also bemade of gas diffusion electrodes which convert hydrogen fed to the anodeside into protons and reduce oxygen on the cathode side into water. In avariant of this type, the electrode/membrane unit may be directlyadjacent to the diluate stream.

[0022] The present invention is explained in greater detail below withreference to illustrative embodiments shown in the attached drawings, inwhich:

[0023]FIG. 1 shows a diagrammatic representation of a first illustrativeembodiment of a fuel cell unit according to the invention having acooling circuit in which an electrode ionization cell is arranged;

[0024]FIG. 2 shows a detailed representation of the electrode ionizationcell from FIG. 1, in which the ion exchanger is in the form of mixed bedpacking;

[0025]FIG. 3 shows a variant of FIG. 2, in which the ion exchanger hasseparate chambers for anion and cation exchanger resins;

[0026]FIG. 4 shows a further variant of FIG. 2, in which a membraneelectrode unit is provided; and

[0027]FIG. 5 shows a diagrammatic representation of a secondillustrative embodiment of a fuel cell unit according to the invention,in which the concentrate stream forms a secondary cooling circuit.

[0028]FIG. 1 shows a diagrammatic view of a fuel cell unit 10 accordingto the invention. The fuel cell unit 10 comprises a fuel cell stack 11,which has feed lines for the fuel 12, for example hydrogen gas, and feedlines for the combustion medium 13, for example air or oxygen. In thecase of feed of gaseous substances, at least one of the supplied gasesis moistened before introduction into the fuel cell stack 11 in order toprevent the polymer membranes from drying out. The reaction products areable to leave the fuel cell stack 11 via outlet lines 14, 15. If thefuel cell is operated with pure hydrogen and oxygen, the reactionproduct formed is water, which can be used partly for moistening thegases flowing in via lines 12 and 13. In the case of the variant shownin FIG. 5 with secondary cooling circuit, another part of the waterformed can also be used for compensation of the water losses which occurin the secondary cooling circuit, as described in greater detail below.The current generated by the fuel cell stack 11 can be fed viacollecting lines 16, 17 to positive or negative connecting terminals 18,19.

[0029] The fuel cell unit 10 has a first cooling circuit, which isdesignated overall with the reference number 20. The coolant used canbe, for example, water, which, depending on the area of application, maycontain further auxiliaries, for example antifreeze agents or corrosioninhibitors. A circulation pump 21 which effects transport of the coolingmedium is arranged in the cooling circuit. Cooling medium is transportedthrough a heat exchanger 22, which is in thermal contact, for example,with ambient air. However, thermal contact with a second cooling circuitmay also be implemented, as described in connection with FIG. 5. Anelectrode ionization cell 23, which reduces the ion concentration in thecooling circuit 20, is arranged downstream of the heat exchanger 22 inthe fuel cell unit according to the invention. The electrode ionizationcell 23 can be operated intermittently. For example, a conductivitysensor 24, which switches, via a switch 25, a direct voltage supplied bya voltage source 26 onto the electrodes of the electrode ionizationcell, can be arranged in the cooling circuit 20.

[0030] The cooling medium of the cooling circuit 20 flows as a so-calleddiluate stream 27 through the electrode ionization cell 23. Ions aredepleted in the diluate stream 27 and enriched in a concentrate stream28, likewise passed through the cell 23. A particular advantage of theelectrode ionization cell is that in operation, regeneration of the ionexchangers, which is preferably arranged in the cell, takes place at thesame time as the deionization of the diluate. Exhaustion of the ionexchanger, as occurs, for example, in the process described in WO00/17951, is avoided in the case of the use proposed in accordance withthe invention of an electrode ionization cell for the deionization. Theenergy expenditure necessary for desalination and regeneration isrelatively low, which means that the cell can also advantageously beoperated continuously. Depending on the input conductivity of thecoolant, initial conductivities of less than 1 μS/cm and even down to0.1 μS/cm can be achieved with a power of less than one watt per literof solution (for comparison, it should be noted that the minimumachievable residual conductivity at the dissociation equilibrium of purewater is about 0.05 μS/cm).

[0031] A very wide variety of electrode ionization cells known per secan be employed in the process according to the invention. The mode offunctioning of an electrode ionization cell and typical illustrativeembodiments of cells of this type are described briefly below withreference to FIGS. 2 to 4.

[0032] In principle, an electrode ionization cell consists of a membranestack in which anion- and cation-permeable ion exchanger membranes arearranged alternately. Parallel flow channels between the membranes areformed by spacers. Every second channel is filled with ion exchangerresin in tight packing. The diluate to be depleted flows through the ionexchanger packing, while the concentrate, in which the concentration ofthe ions removed from the diluate is increased, is passed into channelsin between. The membrane stack is delimited by a pair of electrodes,across which a direct-voltage field is applied transversely.

[0033] In the embodiment shown in FIG. 2, the diluate 27 is passedthrough a plurality of channels 29, 30, each of which is filled with amixed bed of anion and cation exchanger resins. The membrane stack isdelimited by a cathode 31 and an anode 32. The ion exchanger packing isdelimited on the cathode side by a cation exchanger membrane 33, 34 andon the anode side by an anion exchanger membrane 35, 36. The concentratestream 28 is passed between the individual diluate channels 29, 30.Under the influence of the electric field, the ions are transferred fromthe diluate channel into the concentrate channel via the ion exchangerresin and the membranes. The alternating structure of diluate andconcentrate channels enables a greater volume throughput to be achievedfor a given electrode surface area. In the entry region 37, 38 of thediluate into the channels 29, 30, cations are transferred into theconcentrate channel 28 via the cation-permeable membrane 33, 34 andanions via the anion-permeable membrane 35, 36. By contrast,dissociation of water occurs to an increased extent in the outlet region39, 40, the protons and hydroxyl ions formed converting the ionexchangers into the H⁺ and OH⁻ form respectively. The ions liberated inthe inlet region are transported further over the resin surface,ensuring regeneration of the ion exchanger resins at the same time asthe deionization.

[0034] The variant of the electrode ionization cell 23 shown in FIG. 3does not contain a mixed bed, in contrast to the variant in FIG. 2, butinstead the cooling medium for the fuel cell is, as diluate 27, passedfirstly through a cation exchanger resin packing 41 and subsequentlythrough an anion exchanger resin packing 42. In the example shown, thepackings 41 and 42 are in the form of a double layer and are separatedfrom one another by a so-called bipolar membrane 43. At the bipolarmembrane 43, protons are liberated on the side of the cation exchangerresin packing 41 and hydroxyl ions are liberated on the side of theanion exchanger resin packing 42. The cation exchanger resin 41 isdelimited on the side of the cathode 44 by a cation-permeable membrane45, while the anion exchanger resin packing 42 is delimited on the sideof the anode 46 by an anion-permeable membrane 47. The concentrate 28accordingly flows around the resin double layer only on its upper andlower sides.

[0035] In the examples shown, the electrodes can be screened against theconcentrate solution by suitable membranes, so that anodically andcathodically unstable components may also be present in the concentratestream. The cathode can be screened, for example, by an anion-permeablemembrane and the anode by a cation-permeable membrane. The processaccording to the invention and the apparatus according to the inventionare therefore particularly suitable for the deionization of coolants offuel cells to which, owing to their area of application, antifreezeagents have to be added. Thus, the invention is particularly suitablefor applications in the automobile sector, since, for example,water/glycol mixtures can be used as coolant.

[0036] Water diffusing through the membranes is decomposed into hydrogenand oxygen by application of an electric voltage to the gas-evolvingelectrodes.

[0037] A separate cation exchanger packing 48 and anion exchangerpacking 49 are again used in the illustrative embodiment in FIG. 4. Theelectrodes are designed as a membrane/electrode unit. Thus, the anode 50lies directly against the cation exchanger packing 48 with acation-permeable membrane 51 inserted in between, while the cathode 52with an anion-permeable membrane 53 lies against the anion exchangerpacking 49. The concentrate stream 28 is transported in a channelbetween the packings 48 and 49 and is delimited by a cation-permeablemembrane 54 and an anion-permeable membrane 55 respectively.

[0038] The electrodes may be designed as gas diffusion electrodes.

[0039] Finally, FIG. 5 shows a variant of the fuel cell unit 10 in FIG.1, in which the concentrate stream 28 serves as secondary coolingcircuit 56. The components which have already been described inconnection with the variant in FIG. 1 are denoted by the same referencenumerals and are not explained in greater detail here. The concentratestream 28 takes up heat from the diluate stream 27 in the heat exchanger22. The cooled diluate stream 27 is fed to the electrode ionization cell23. The warmed concentrate stream is passed firstly through a primarycooler 57 and, after cooling, is likewise fed to the electrodeionization cell. A conveying pump 58 is arranged in the secondarycooling circuit 56. Water losses in the secondary cooling circuit 56 canbe replaced as needed via line 59. If the fuel cell is fed with purehydrogen and oxygen, some of the water formed as reaction product can befed into the secondary cooling circuit 56 from lines 14, 15 via line 59.

We claim:
 1. A process for the deionization of a fuel cell coolingmedium circulating in a first cooling circuit, in which the coolingmedium is subjected to at least intermittent electrochemicaldeionization.
 2. A process as claimed in claim 1, wherein the coolingmedium is deionized continuously.
 3. A process as claimed in either ofclaims 1 and 2, wherein the cooling medium is passed as diluate streamthrough an electrode ionization cell during the deionization.
 4. Aprocess as claimed in claim 3, wherein the diluate stream is cooledbefore the deionization.
 5. A process as claimed in claim 4, wherein aconcentrate stream from the electrode ionization cell is cooled in asecond cooling circuit and subsequently used for cooling the diluatestream.
 6. The use of an electrode ionization cell for the deionizationof a fuel cell cooling medium.
 7. A fuel cell unit (10) having at leastone fuel cell (11) and a first cooling circuit (20) for the fuel cell,wherein at least one electrode ionization cell (23), through which adiluate stream (27) serving as cooling medium and a concentrate stream(28) flow, is arranged in the cooling circuit (20).
 8. A fuel cell unitas claimed in claim 7, wherein the electrode ionization cell (23)comprises a mixed bed (29, 30) comprising anion and cation exchangerresins through which the diluate stream (27) flows.
 9. A fuel cell unitas claimed in claim 7, wherein the electrode ionization cell comprises afirst chamber containing a cation exchanger resin (41; 48), throughwhich the diluate stream (27) flows, and a second chamber containing ananion exchanger resin (42; 49).
 10. A fuel cell unit as claimed in claim9, wherein a bipolar membrane (43) is arranged between the first chamberand the second chamber.
 11. A fuel cell unit as claimed in any one ofclaims 7 to 10, wherein the concentrate stream (28) is circulated in asecond cooling circuit (56), with a heat exchanger (22) which thermallycouples the first and second cooling circuits (20, 56), being arrangedupstream of the electrode ionization chamber (23).